Post Office Protocol 3

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POP3 was developed in response to SMTP and is designed for workstations that don't have the resources to maintain both SMTP services and a message transfer system. In addition, continuous connectivity to the network for each workstation, which is necessary for an SMTP host to operate correctly, is impractical in many instances.

POP3 permits a workstation to dynamically access a server that is holding mail for it. It does not allow extensive manipulation of mail on the server. Instead, it is used to download mail from the server. Once the mail has been downloaded, the server deletes its copy of the messages unless you have set your POP3 client to keep a copy of the message on the server. POP3 is a very small, fast, lean protocol that is really for mail retrieval only. To send mail, a POP3 client uses a normal SMTP connection to the destination mail server or a local SMTP relay server.

POP3 has both a client side and a server side. The server starts the POP3 service by listening on TCP port 110. When a POP3 client wants to use this service, it establishes a TCP connection with the server and then receives a greeting from the server. The client and server then exchange commands and responses until the connection is either closed or aborted. Like SMTP commands, POP3 commands are not case sensitive and can contain one or more arguments. A POP3 session between the server and the client will progress through several stages:

1. Once the TCP connection has been opened and the POP3 server has sent a greeting, the session enters the Authorization state. In this state, the client must identify itself to the POP3 server.
2. Once the client has successfully been authenticated, the session enters the Transaction state. During this phase, the server gathers the client's mail and, in response to requests from the client, sends mail to the client. The client's mailbox is locked to prevent messages from being modified or removed until the session enters the Update state. A series of commands and responses usually passes between the client and server during this phase.
3. When the client issues a QUIT command, the session enters the Update state. In this state, the POP3 server releases any resources it is holding on behalf of the client and sends a good-bye message. Messages are then deleted from the server, and the TCP session is terminated.

Table 17-3 summarizes the POP3 commands.

Table 17-3. Summary of POP3 commands

Command	Description
USER	Supplies user name for mailbox
PASS	Supplies password for mailbox
STAT	Requests the number of messages and total size of message
LIST	Lists the index and size of all messages
RETR	Retrieves the specified messages
DELE	Deletes the specified message
NOOP	No action required
RSET	Rolls back message deletion
QUIT	Updates (commit) message deletion and terminates connection

Administering POP3 in Exchange 2000 Server simply involves choosing the number of users that can connect to each POP3 virtual server, indicating whether POP3 is assigned to a specific IP address or All Unassigned, and setting message encoding instructions for the virtual server. All of these settings in the POP3 virtual server work the same as in other protocols and are described throughout this chapter.